GEORGETOWN, Colo. (AP) — Federal investigators are trying to determine why an avalanche shell exploded Monday while state workers were trying to fire a round up a hillside to clear chasms filled with snow near Interstate-70.

Colorado Department of Transportation spokeswoman Crystal Morgan said Tuesday a transportation department worker is in serious condition, and an avalanche forecaster is in fair condition. The federal Bureau of Alcohol, Tobacco, Firearms and Explosives is investigating.

The two men were standing behind a barricade in the area about 50 miles west of Denver when the shell detonated prematurely, injuring the men above the waist.

Their names have not been released.

The shells are fired using launchers with compressed gas up mountainsides to set off controlled avalanches to protect drivers and backcountry skiers.